GPS and Speed data overlay on videos is not accurate

Since unboxed from new, the RCT715 GPS and Speed data overlay in videos are just props. They don't work at all. It will pick up random speed and gps coordinates at the beginning and it will stick there until the end of the ride, if I am lucky, otherwise, it will just say 0 kph as speed and some random coordinates from any point of the ride. The app says if you have a compatible Garmin device connected, it will be used as the source of speed and gps data. I have an 830 and using speed sensor but still doesn't cut it to get the gps and speed data work properly.

Am I missing something?